Silk is traditionally considered either a fiber used in textiles or a biomedical scaffold but not a structural solid with potential to compete with engineered composites. But this is changing. Scientists from Tufts University, Imperial College London, and the University of Michigan demonstrated that natural silk fibers could be converted into a high-performance bulk material through the process of fusing without dissolving the fibers or using synthetic binders. Instead of rebuilding the material from extracted proteins, the researchers aligned natural fibers obtained from silkworm cocoons, stripped off sericin, and consolidated them via application of heat and pressure. The resulting material, described in Nature Sustainability, retains most of the internal architecture of silk fiber rather than destroying it during processing.
It makes a difference since the performance of silk is highly dependent on its structure. In natural fibers, the balance between stiffness provided by crystalline regions and flexibility of amorphous regions is essential for strength and energy dissipation. “The silk is like a composite,” said David Kaplan, Stern Family Endowed Professor of Engineering at Tufts. “There is a more mobile, amorphous phase of the fiber proteins, and there is the part of the protein chain that folds to form sheet-like surfaces that stack up into crystalline structures.” The innovative method helps preserve more structure, which is the reason for the remarkable performance of the fused material that surpasses wood and bone in terms of tensile toughness, approaches Kevlar, and demonstrates ballistic impact resistance superior to some carbon-fiber reinforced polymer composites.
Processing conditions are critical since the optimal consolidation was achieved at 125-215 degrees Celsius and pressures ranging from hundreds to thousands of atmospheres. If not enough energy is applied, there is poor bonding of fibers, while excess energy leads to brittleness. It illustrates one of the fundamental concepts of biomimetics: performance is achieved less via use of extraordinary materials but thanks to retention and organization of complex architecture, which is hardly achievable with conventional methods. Reviews on hierarchical biological composites repeatedly demonstrated that wood, bone, nacre, and silk achieve unique properties due to their specific structure, which cannot be easily replicated by manufacturing processes.
Silk provides another benefit that is rarely encountered among other materials used in structural applications. Being naturally biocompatible, it can be programmed to degrade over time. Thus, the material gets a second life, which goes beyond simple comparisons of mechanical properties. The researchers discovered that denser and less dense forms of fused silk degrade at different speeds, which means that the mechanical lifetime can be adjusted depending on requirements for implants or fixation devices. A separate study on Silk I crystalline materials revealed the reasons why it is beneficial: while some silk structures are stable in water, sterilization with alcohol, and long-term storage, they are subject to enzymatic degradation once exposed to biological environment.
When it comes to orthopedic plates, screws, and other fixation devices, there are few alternatives to this material. The research suggests more applications for the novel biomaterial. Scientists from the University of Michigan showed that fused silk is capable of polarizing terahertz radiation, which is relevant for sensing and communications applications. Since biomimetic materials tend to become multifunctional, the optical properties of the material might prove even more important than its mechanical characteristics. Silk is unlikely to replace carbon fiber and Kevlar in aerospace and industrial applications anytime soon. However, the article shows that silk can be used to produce solid materials whose performance is dictated by biological architecture and not petrochemical additives.